1. Field of the Invention
The present invention relates generally to circuit board structures and fabrication methods thereof, and more particularly, to a circuit board structure and fabrication method thereof capable of improving the bonding between the dielectric layer and the circuit layer of the circuit board structure.
2. Description of Related Art
High-density multi-layer circuit boards with fine circuit have become a main trend of future development, wherein the quality of circuit layers of the circuit boards for signal transmission is an important factor in electrical performance of the circuit boards.
With the line-to-space ratio (L/S) decreasing, circuit boards nowadays are subject to a slight change of shape or dimensions as far as electrical quality is concerned, especially when circuit boards operate at a high-frequency state.
In addition, in a conventional circuit build-up process, the surface of a dielectric layer is roughened before a circuit layer is formed so as to increase the bonding force between the dielectric layer and the circuit layer. However, the roughening process is difficult to control. If the surface of the dielectric layer is not efficiently roughened, the circuit layer can easily detach from the dielectric layer. As for circuit boards having fine circuit, the roughening process becomes much more difficult as an efficient bonding area between the fine circuit and the dielectric layer is reduced. On the other hand, the surface of the circuit layer can be roughened by etching in the conventional fabrication process so as to improve the bonding between the circuit layer and the dielectric layer, thereby preventing delamination of the dielectric layer from the circuit board. As shown in FIGS. 1A to 1C, a circuit board 11 is provided, which has a circuit layer 12 formed on at least one surface thereof The circuit layer 12 has a plurality of electrically connecting pads 121, as shown in FIG. 1A. Then, the surface of the circuit layer 12 is roughened by chemical etching so as to form a roughened surface 12a on the circuit layer 12, as shown in FIG. 1B. Thereafter, a dielectric layer 13 or a solder mask layer is formed on surfaces of the circuit board 11 and the circuit layer 12, as shown in FIG. 1C. The bonding force between the circuit layer 12 and the dielectric layer 13 (or the solder mask layer) is strengthened through the roughened surface 12a of the circuit layer 12, thus preventing the detachment of the uncovered dielectric layer 13 or solder mask layer from the circuit layer 12.
However, it is difficult to achieve a uniformly etched surface, and any slight change of shape or dimensions of the circuit layer can affect the electrical quality. The conventional roughening process often ends up with uneven thickness and uncontrollable line width of a circuit layer, especially the circuit layer of high-density circuit boards, which affects the electrical quality of the circuit boards and makes it difficult to be applied in circuit boards having a fine circuit. For circuit boards with a high-density layout and a small line width, a badly timed etching process is likely to sever a circuit and result in an open circuit, or tends to undercut the circuit layer and therefore taper the cross-section of the circuit layer downward and reduce the cross-sectional area of the circuit layer, and in consequence the signal transmission quality and electrical performance of the circuit board are adversely affected. On the other hand, an etching process may be too short to roughen the surface of the circuit layer sufficiently, such that the dielectric layer formed on the circuit layer readily detaches from the circuit layer, thereby resulting in poor bonding between the circuit layer and the dielectric layer and adversely affecting the quality of products.
Therefore, there is an urgent need to provide a circuit board structure and fabrication method thereof so as to overcome the above-described drawbacks.